INVESTIGATING THE ROLE OF REHYDRATION METHOD ON SUBSEQUENT OM-NANOSCALE GOETHITE INTERACTIONS USING CHROMATE AS A CHEMICAL PROBE

Interactions between organic matter (OM) and nanoscale minerals continue to be researched due to the complexity of the interactions between these materials, and the significant impact that OM has on the environmental fate of nanoscale minerals. While several factors (pH, ionic strength, etc.) influence the interaction between mineral surfaces and OM, in this work we investigated if the conditions under which organic matter was rehydrated could impact subsequent mineral-OM interactions.

Commercially available Suwannee River Natural Organic Matter (SWNOM) was rehydrated under various pH and ionic strength conditions (distilled-deionized water, 0.1 mM NaOH, and 10 mM NaHCO₃). The OM solutions were reacted with goethite nanoparticles for either ~2 hours or ~24 hours prior to the addition of chromate. The amount of chromium sorption was compared to results from a baseline experiment done without organic matter added to the system.

Results from the metal sorption experiments confirmed that chromate was readily sorbed onto the surface of goethite in the absence of SWNOM. Goethite nanoparticles exposed to SWNOM for ~2 hours prior to the addition of chromate saw an average of a ~26% reduction in the initial amount of chromate sorbed compared to the baseline results without SWNOM, with little difference between the experiments using different rehydration methods (24%, 29%, and 24% respectively). When goethite-SWNOM contact time was increased to ~24 hours prior, the decrease in initial chromate sorption increased to an average of ~48%, with a larger difference between the experiments using different rehydration methods (53%, 43%, and 49% respectively).

The results of this investigation revealed that when the mineral-OM contact time prior to chromate addition was short, the SWNOM rehydration method had no significant impact on chromate sorption. However, when the mineral-OM contact time was increased to ~24 hours, the amount of chromate sorbed varied based on the SWNOM rehydration method. The exact reason for this variation is currently unknown, but is likely related to the heterogeneous nature of the SWNOM and the fractionation of the organic compounds onto the mineral surface as mineral-OM contact time increases. These results highlight the complexity of mineral-OM interactions.